title: HDAC inhibitor entinostat restores responsiveness of letrozole-resistant MCF-7Ca xenografts to aromatase inhibitors through modulation of Her-2.
abstract: We previously showed that in innately resistant tumors, silencing of the estrogen receptor (ER) could be reversed by treatment with a histone deacetylase (HDAC) inhibitor, entinostat. Tumors were then responsive to aromatase inhibitor (AI) letrozole. Here, we investigated whether ER in the acquired letrozole-resistant tumors could be restored with entinostat. Ovariectomized athymic mice were inoculated with MCF-7Ca cells, supplemented with androstenedione (Δ(4)A), the aromatizable substrate. When the tumors reached about 300 mm(3), the mice were treated with letrozole. After initial response to letrozole, the tumors eventually became resistant (doubled their initial volume). The mice then were grouped to receive letrozole, exemestane (250 μg/d), entinostat (50 μg/d), or the combination of entinostat with letrozole or exemestane for 26 weeks. The growth rates of tumors of mice treated with the combination of entinostat with letrozole or exemestane were significantly slower than with the single agent (P < 0.05). Analysis of the letrozole-resistant tumors showed entinostat increased ERα expression and aromatase activity but downregulated Her-2, p-Her-2, p-MAPK, and p-Akt. However, the mechanism of action of entinostat in reversing acquired resistance did not involve epigenetic silencing but rather included posttranslational as well as transcriptional modulation of Her-2. Entinostat treatment reduced the association of the Her-2 protein with HSP-90, possibly by reducing the stability of Her-2 protein. In addition, entinostat also reduced Her-2 mRNA levels and its stability. Our results suggest that the HDAC inhibitor may reverse letrozole resistance in cells and tumors by modulating Her-2 expression and activity.
authors.affiliation: Corresponding Author: Gauri J. Sabnis, Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, 685 W. Baltimore St, HSF-I 580, Baltimore, MD 21201. gsabn001@umaryland.edu.
authors.forename: Gauri J
authors.initials: GJ
authors.lastname: Sabnis
authors.forename: Olga G
authors.initials: OG
authors.lastname: Goloubeva
authors.forename: Armina A
authors.initials: AA
authors.lastname: Kazi
authors.forename: Preeti
authors.initials: P
authors.lastname: Shah
authors.forename: Angela H
authors.initials: AH
authors.lastname: Brodie
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000008:administration & dosage / Q000494:pharmacology*; D001141:Aromatase / Q000235:genetics / Q000378:metabolism; D047072:Aromatase Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D001549:Benzamides / Q000008:administration & dosage / Q000494:pharmacology*; D004195:Disease Models, Animal; D019008:Drug Resistance, Neoplasm; D047628:Estrogen Receptor alpha / Q000235:genetics / Q000378:metabolism; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism; D056572:Histone Deacetylase Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D006801:Humans; D000077289:Letrozole; D020935:MAP Kinase Signaling System / Q000187:drug effects; D061986:MCF-7 Cells; D051379:Mice; D009570:Nitriles / Q000008:administration & dosage / Q000494:pharmacology*; D011725:Pyridines / Q000008:administration & dosage / Q000494:pharmacology*; D020871:RNA Stability / Q000187:drug effects; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D014230:Triazoles / Q000008:administration & dosage / Q000494:pharmacology*; D047368:Tumor Burden / Q000187:drug effects; D023041:Xenograft Model Antitumor Assays
chemical_list: D000970:Antineoplastic Agents; D001141:Aromatase; D047072:Aromatase Inhibitors; D001549:Benzamides; D047628:Estrogen Receptor alpha; D018841:HSP90 Heat-Shock Proteins; D056572:Histone Deacetylase Inhibitors; D000077289:Letrozole; D009570:Nitriles; D011725:Pyridines; D018719:Erb-b2 Receptor Tyrosine Kinases; D014230:Triazoles; C118739:entinostat